Veterinary Services

Progressive Retinal Atrophy in the Tibetan TerrieR

Progressive Retinal Atrophy (PRA) is a well-recognised inherited condition that many breeds of dog are predisposed to. The condition is characterised by bilateral degeneration of the retina which causes progressive vision loss that culminates in total blindness. There is no treatment for PRA, of which several genetically distinct forms are recognised, each caused by a different mutation in a specific gene. The various forms of PRA are typically breed-specific, with clinically affected dogs of the same breed usually sharing an identical mutation. Clinically affected dogs of different breeds, however, usually have different mutations, although PRA-mutations can be shared by several breeds.

PRA DNA test available to Tibetan terriers In 2011 geneticists working in the Kennel Club Genetics Centre at the Animal Health Trust identified a recessive mutation that is associated with the development of Late Onset Progressive Retinal Atrophy (LOPRA) in the Gordon Setter. Owners of Gordon Setters with LOPRA report that their affected dogs develop night blindness in the first instance, which is indicative of a rod-cone degeneration, so we termed this mutation rcd4 (for rod-cone degeneration 4) to distinguish it from other, previously described, forms of rod-cone degeneration.

Following our work in the Gordon Setter we have found the rcd4 mutation in PRA affected dogs of other breeds, including the Irish and English setter and now also the Tibetan terrier. As a result the AHT will make the rcd4 DNA test available to Tibetan terriers, from October 2nd, 2012. The DNA test we are offering examines the DNA from each dog being tested for the presence or absence of this precise mutation and is thus a ‘mutation-based test’ and not a ‘linkage-based test’.

We have tested DNA from 11 Tibetan terriers affected with PRA and found 2 of them were homozygous (carried two copies) for the rcd4 mutation. This indicates that there is at least one additional, genetically distinct, form of PRA segregating in the Tibetan terrier, that is caused by an as yet unidentified mutation and that this additional mutation(s) is probably more common than the rcd4 mutation. At the time of writing (September 2012) we are investigating the frequency of the rcd4 mutation in a random subset of Tibetan terriers and expect to be able to update the Tibetan terrier community with the results by the end of October 2012. It is important for owners to appreciate that the rcd4 DNA test detects the rcd4 mutation only and cannot provide any information regarding the additional, currently unknown PRA mutation(s).

Rcd4 DNA Test

Breeders using the rcd4 DNA test will be sent results identifying their dog as belonging to one of three categories. In all cases the terms ‘normal’ and ‘mutation’ refer to the position in the DNA where the rcd4 mutation is located; it is not possible to learn anything about any other region of DNA from the rcd4 DNA test.

CLEAR: these dogs have two normal copies of DNA. Clear dogs will not develop PRA as a result of the rcd4 mutation, although we cannot exclude the possibility they might develop PRA due to other mutations they might carry that are not detected by this test.

CARRIER: these dogs have one copy of the mutation and one normal copy of DNA. These dogs will not develop PRA themselves as a result of the rcd4 but they will pass the mutation on to approximately 50% of their offspring. We cannot exclude the possibility that carriers might develop PRA due to other mutations they might carry that are not detected by this test.

GENETICALLY AFFECTED: these dogs have two copies of the rcd4 mutation and will almost certainly develop PRA during their lifetime. The average age of diagnosis for dogs with rcd4 is 10 yo, although there is considerable variation within the breed.

AdviceThe rcd4 mutation is recessive which means that all dogs can be bred from safely but carriers and genetically affected dogs should only be bred to DNA tested, clear dogs. About half the puppies from any litter that has a carrier parent will themselves be carriers and any dogs from such litters that will be used for breeding should themselves be DNA tested prior to breeding so appropriate mates can be selected. All puppies that have a genetically affected parent will be carriers.

It is advisable for all breeding dogs to have their eyes clinically examined by a veterinary ophthalmologist prior to breeding and throughout their lives so that any cases of PRA caused by additional mutations can be detected and that newly emerging conditions can be identified.

The Animal Health Trust is a veterinary charity, based in Suffolk, which offers clinical referral services and diagnostic testing for horses, dogs and cats in East Anglia, across the UK and internationally. Through its research programmes it develops new diagnostic tests, treatments and vaccines to help thousands of animals.